1. Field of the Invention
The present invention relates to a negative electrode for lithium secondary battery and a manufacturing method thereof, and to a lithium secondary battery.
2. Related Art
Conventionally, the following negative electrodes for lithium secondary battery have been proposed in Japanese Patent Publication No. 2948205 and Japanese Patent Laid Open No. 11-339777. The former negative electrode has been manufactured by sintering silicon or silicon/carbon compound applied on a metallic substrate. The latter negative electrode has been manufactured by sintering a complex of silicon and conductive carbon or conductive metal together with a conductive metallic foil. According to the above conventional methods, it is possible to manufacture a negative electrode which is excellent in conductivity due to a reduction of a contact resistance between a sintered body containing an active material and a substrate.
However, an active material containing silicon has an extremely high expansion and shrinkage coefficient by charge and discharge. Further, when charge-discharge cycle is repeated, the active material separates from a current collector, and current collection is reduced, therefore sufficient cycle characteristics can not be obtained.
An object of the present invention is to provide a negative electrode for lithium secondary battery, which is excellent in charge-discharge cycle characteristics, and a manufacturing method thereof, and to provide a lithium secondary battery.
A negative electrode for lithium secondary battery according to the present invention is an electrode obtained by sintering a mixture of an active material alloy and a binder arranged on a current collector made of metallic foil, or sintering a mixture of the active material alloy, conductive metal powder and a binder arranged on a current collector made of metallic foil, and is characterized in that the active material alloy after sintering is substantially amorphous.
In the negative electrode for lithium secondary battery of the present invention, the active material alloy is sintered; therefore, a binding strength between the active material alloy particles is large. Further, the active material alloy is substantially amorphous; therefore, it is possible to absorb and release lithium without causing a great change of crystal structure during charge and discharge. Accordingly, in the negative electrode for lithium secondary battery of the present invention, the active material particles are hardly pulverized and detached from the current collector when the active material is expanded and shrunk by absorption and release of lithium on charge and discharge. Therefore, it is possible to improve charge-discharge cycle characteristics.
The manufacturing method of a negative electrode for lithium secondary battery of the present invention is characterized in that a mixture of an active material alloy which is substantially amorphous and a binder, or a mixture of the active material alloy, conductive metal powder and a binder is arranged on a current collector, and then sintered under a condition such that the active material alloy after sintering is substantially amorphous.
In the manufacturing method of the present invention, the mixture is sintered under condition such that the active material alloy after sintered is substantially amorphous. More specifically, for example, heat treatment is carried out at a temperature lower than the crystallization temperature of the active material alloy, and thereby, the active material alloy can be sintered in a substantially amorphous state. The crystallization temperature of the active material alloy can be measured by, for example, DSC (differential scanning calorimeter).
Further, it is preferable to carry out heat treatment for sintering in a non-oxidizing atmosphere. The heat treatment in the non-oxidizing atmosphere can be carried out in a vacuum or in an inert gas atmosphere such as argon. Further, the heat treatment can be carried out in a reducing atmosphere such as hydrogen atmosphere. A discharge plasma sintering method and hot press method may be employed as the sintering method.
According to the present invention, in order to arrange the mixture of the active material alloy and the binder, or the mixture of the active material alloy, the conductive metal powder and the binder, on the current collector, the slurry of these mixtures can be applied on the current collector and then dried. More specifically, a slurry is prepared by mixing the active material alloy or the active material alloy and the conductive metal powder with a solution of the binder, and the obtained slurry is applied onto the current collector and then dried.
Further, after application and drying, it is preferable that the mixture layer is rolled together with the current collector before sintering. Because of such rolling, it is possible to improve a packing density of the mixture layer, and to improve an adhesion between active material particles and an adhesion of the active material particles to the current collector.
The negative electrode for the lithium secondary battery of the present invention is not limited to the electrode manufactured by the above manufacturing method of the present invention.
In the present invention, the words xe2x80x9csubstantially amorphousxe2x80x9d means that existence of a halo portion in X-ray diffraction profile is observed, and a degree of non-crystallinity defined by the following equation is 0.3 or more.
Degree of non-crystallinity=maximum peak strength of halo portion profile/maximum peak strength of entire profile
FIG. 2 is a diagram to explain the maximum peak strengths of entire profile and halo portion profile in X-ray diffraction profile. As shown in FIG. 2, the maximum peak strength of entire profile is determined from the height of the highest peak of the entire profile from the base line. On the other hand, the maximum peak strength of halo portion is determined from the height of the highest peak of the halo portion from the base line.
In the manufacturing method of the present invention, the active material alloy which is substantially amorphous is used. The substantially amorphous active material alloy is prepared by liquid quenching method, vacuum evaporation method, ion plating method, mechanical alloying method or the like. In these methods, the liquid quenching method is preferable for preparing a large amount of amorphous alloy at a low cost. The liquid quenching method is a rapid solidification method including; single roll and twin roll methods of making an alloy molten and injecting the molten alloy onto a copper roll rotating at high speed; a gas atomization method of spraying the molten alloy using an inert gas; a water atomization method of spraying the molten alloy using water; and a gas-spraying water atomization method of spraying molten alloy using gas and then cooling it using water.
The active material alloy used in the present invention contains preferably Si, further preferably Al, Si and transition metal. The alloy containing Al, Si and transition metal is readily prepared as an amorphous alloy by the above-mentioned liquid quenching method. Examples of the transition metal are chromium, manganese, iron, cobalt, nickel and the like.
Preferably, a metallic foil having a surface roughness Ra of 0.2 xcexcm or more is used as the current collector in the present invention. The surface roughness Ra in the present invention is a value before sintering. The metallic foil having the above surface roughness Ra provides a larger contact area between the active material particles and the surface of the metallic foil, which improves current collection. Further, the larger contact area provides effective sintering and greatly improved adhesion between the current collector and the active material particles. The above surface roughness Ra is determined by Japanese Industrial Standard (JIS B 0601-1994), and is measured by, for example, a surface roughness meter.
The upper limit of the surface roughness Ra of the current collector is not specifically limited, however, preferably 10 xcexcm or less, because the current collector has a thickness of 10 to 100 xcexcm.
As the current collector for the present invention, used is a foil made of metal such as copper, nickel, iron, titanium, or cobalt, or alloy of at least one of these metals. It is particularly preferable that a copper foil is used. As described above, it is preferable that the surface roughness Ra is 0.2 xcexcm or more; therefore, for example, an electrolytic copper foil is preferably used as the copper foil. The electrolytic copper foil is prepared in such a manner that copper is deposited on the surface of copper foil by electrolytic method. Further, other metallic foil having a copper layer formed on the surface by electrolysis may be used.
According to the present invention, as described above, the conductive metal powder is mixed with the active material alloy, if necessary. By mixing the conductive metal powder, a firmly conductive network made of the conductive metal powder can be formed around the active material particles. Therefore, it is possible to improve current collection. The same material as the above current collector may be used as the material of the conductive metal powder. For example, powder made of metal such as copper, nickel, iron, titanium, or cobalt, or alloy of at least one of these metals may be used. In particular, copper or copper alloy powder is preferably used as the conductive metal powder.
Moreover, a mean particle diameter of the active material alloy particle used in the present invention is not specifically limited. However, in order to generate effective sintering, the mean particle diameter is preferably 100 xcexcm or less, and further preferably 50 xcexcm or less, and most preferably 10 xcexcm or less. The mean particle diameter of active material particles is smaller, more excellent cycle characteristics is obtained. Further, a mean particle diameter of the conductive metal powder used in the present invention is not limited. However, it is preferably 100 xcexcm or less, further preferably 50 xcexcm or less, and further preferably 10 xcexcm or less.
In the present invention, a mixing ratio of the conductive metal powder is preferably in a range from 0.05 to 50 parts by weight with respect to 1 part by weight of active material particle. When the mixing ratio of the conductive metal powder is too less, excellent charge-discharge cycle characteristic may not be obtained. On the other hand, when the mixing ratio is too much, the mixing ratio of active material particles is relatively reduced, so that charge-discharge capacity becomes small.
In the present invention, the thickness of the metallic foil is not specifically limited, but preferably falls in the range of 10 xcexcm to 100 xcexcm. Further, the thickness of the sintered body, which is formed by sintering the mixture layer of the active material particles and the conductive metal powder on the metallic foil or sintering the active material particles on the metallic foil, is not specifically limited. However, the thickness of the sintered body is preferably 1000 xcexcm or less, and further preferably 10 xcexcm to 100 xcexcm.
The binder used in the present invention is not specially limited so far as it can be used for the electrode of lithium secondary battery in general. More specifically, a fluorine-containing binder such as poly vinylidene fluoride may be used.
A lithium secondary battery of the present invention comprises the negative electrode of the present invention or the negative electrode manufactured by the method of the present invention, a positive electrode containing positive active material, and non-aqueous electrolyte.
The electrolyte solvent for use in the lithium secondary battery of the present invention is not particularly limited in type but can be exemplified by a mixed solvent which contains cyclic carbonate such as ethylene carbonate, propylene carbonate or butylene carbonate and also contains chain carbonate such as dimethyl carbonate, methyl ethyl carbonate or diethyl carbonate. Also applicable is a mixed solvent of the above-listed cyclic carbonate and an ether solvent such as 1,2-dimethoxyethane or 1,2-diethoxyethane. Examples of electrolyte solutes include LiPF6, LiBF4, LiCF3SO3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(CF3SO2) (C4F9SO2), LiC(CF3SO2)3, LiC(C2F5SO2)3 and mixtures thereof. Other applicable electrolytes include, for example, a gelled polymer electrolyte comprising an electrolyte solution impregnated into a polymer electrolyte such as polyethylene oxide or polyacrylonitrile and inorganic solid electrolytes such as LiI and Li3N. The electrolyte for the recharageable lithium battery of the present invention can be used without limitation, so long as an Li compound as its solute that imparts an ionic conductivity, as well as its solvent that dissolves and retains the Li compound, remain undecomposed at voltages applied during charge, discharge and storage of the battery.
The positive active material for use in the lithium secondary battery of the present invention is exemplified by lithium-containing transition metal oxides such as LiCoO2, LiNiO2, LiMn2O4, LiMnO2, LiCo0.5Ni0.5O2 and LiNi0.7Co0.2Mn0.1O2; lithium-free metal oxides such as MnO2; and the like. Other substances can also be used, without limitation, if they are capable of electrochemical insersion and release of lithium.